Ortho/para hydrogen converter for rapid deposition matrix isolation spectroscopy

We describe the construction and operation of a catalyst-bed type device for precooling and equilibrating the ortho/para composition of a hydrogen gas flow. We use this device to vapor deposit millimeters thick cryogenic parahydrogen ( pH(2)) solids which are remarkably transparent [M. E. Fajardo and S. Tam, J. Chem. Phys. 108, 4237 (1998)]. Infrared absorption spectra of solids deposited at pH(2) flow rates up to 290 mmol/h (solid thickness growth rates up to 75 mu m/min indicate a residual orthohydrogen (oH(2)) content of less than 0.01%. Gas phase thermal conductivity measurements indicate a residual oH(2) content of 0((+5)(-0))% for flow rates up to 1.9 mol/h. These pH(2) solids can be doped readily by simple codeposition of various impurities produced by any of the numerous dopant sources developed in previous matrix isolation spectroscopy (MIS) studies. The long achievable path lengths, and the desirable properties of pH(2) as a matrix host, will enable significant new fundamental and practical applications for MIS. [S0034-6748(99)00404-9].